1. Field of the Invention
This invention relates to a readable optical disc from which the data recorded on the optical disc can be read even when there are areas of missing data caused by black dots or the like.
2. Description of the Related Art
In recent years, the so-called DVD (Digital Versatile Disc) has become widely used and is becoming the norm as a disc-shaped data-recording medium that is capable of high-density data recording.
Conventionally, a control-data area is formed on an optical disc such as a DVD for recording beforehand ID data for the optical disc and control data for the contents data that are recorded on the optical disc. Also, when reproducing the data on the optical disc, the ID data and control data recorded in this control area are read in order to perform reproduction control of the contents data.
Moreover, the optical disc is such that it is irradiated by a laser beam emitted from the optical head of the reproduction apparatus. In this way, the pits recorded on the optical disc are traced, and the contents data, ID data and control data are read.
With this kind of optical disc, when there are flaws or black dots on the disc, the optical head of the reproduction apparatus jumps and the same location on the disc is repeatedly reproduced, or it becomes impossible to read data recorded in a black dot.
Therefore, recently, control of the tracking is performed when reproducing the data on the optical disc. This control of the tracking is called tracking control.
The control data is particularly important data when reproducing the data recorded on the optical disc. Therefore, as a recent method of reproducing data on the optical disc, reproduction failure and the inability to read data are prevented by performing not only the tracking control described above, but also by encoding the control data using an error-correction signal or Wobble signal.
Therefore, by using an error-correction signal or Wobble signal in addition to performing tracking control for the optical disc, errors are corrected by guessing the correct data from other read data as Error-Correcting, and the control data can be read accurately even when it is not possible to read part of the control data due to black dots.
However, in the case of a reproduction-only optical disc such as a DVD on which data are recorded with high density, the data on the optical disc are reproduced while detecting the pits that represent data. Therefore, when a missing area occurs due to flaws or black dots on the optical disc, that missing area becomes the source of a tracking error, and there is a possibility that it will be impossible to read the data recorded in that missing area.
In other words, at the normal NA of the objective lens for an optical disc (NA (Numerical Aperture: Opening number)=0.5) there is hardly any effect on reproduction due to tracking control or the Error Correction even when there are black dots. However, in the case of reading data from an optical disc having a high recording density, an objective lens having a high NA is used, so when there are areas of missing data due to black dots and when those areas of missing data are large, it becomes impossible to read data recorded in the areas of missing data using error correction or the Wobble method.